commit 342fdad21f737bde2399231d772477d1b2221fe1
from: Stefan Sperling <stsp@stsp.name>
via: Thomas Adam <thomas@xteddy.org>
date: Tue Mar 19 13:37:06 2024 UTC

avoid a rename/stat race when gotd installs a new pack and then uses it

Reset the cached repository's pack directory mtime after installing a new
pack and pack index file. I have observed the mtime of the pack directory
as reported by stat(2) remaining unchanged, until some time has passed
beyond the rename(2) calls used to install the pack file and its index.

If gotd immediately tries to read objects installed in a new pack file then
the mtime reported by stat(2) might appear as unchanged. gotd will then fail
to update its cached list of pack index files and not find the newly
installed objects.
Clearing the cached timestamp forces a readdir(3) call which does expose
the newly installed pack index file as expected.

Not sure whether stat(2) is supposed to immediately expose mtime changes
after a rename(2). If so then this might warrant digging into the kernel.

Seen while running regression tests for upcoming gotd notification support.

commit - e8c5d4f87401e77adfdec88437f3b973e7b47262
commit + 342fdad21f737bde2399231d772477d1b2221fe1
blob - bc9ef0d64141e9df96d93d992d4b70ec9ccf4f2f
blob + 27ecb3cb1f2dfa87ae9b75649146a015b1e45941
--- gotd/repo_write.c
+++ gotd/repo_write.c
@@ -1713,6 +1713,12 @@ repo_write_dispatch_session(int fd, short event, void 
 					    err->msg);
 					break;
 				}
+				/*
+				 * Ensure we re-read the pack index list
+				 * upon next access.
+				 */
+				repo_write.repo->pack_path_mtime.tv_sec = 0;
+				repo_write.repo->pack_path_mtime.tv_nsec = 0;
 			}
 			err = update_refs(iev);
 			if (err) {
blob - ebe568d9449b25eac52742c96be60d186d6244e8
blob + 9754b5ea953bf492a5912577904d94d66b1426a8
--- gotd/session.c
+++ gotd/session.c
@@ -367,6 +367,10 @@ install_pack(struct gotd_session_client *client, const
 		    packidx_path);
 		goto done;
 	}
+
+	/* Ensure we re-read the pack index list upon next access. */
+	gotd_session.repo->pack_path_mtime.tv_sec = 0;
+	gotd_session.repo->pack_path_mtime.tv_nsec = 0;
 
 	free(client->packidx_path);
 	client->packidx_path = NULL;